The motors are hypergolic, meaning that they spontaneously ignite when their two fuels mix. Unlike a solid rocket motor that has to burn fully like a firework, this allows them to be switched on and off at will. Another advantage is that the fuels don't need to be cooled cryogenically.

In the event of an accident, however, volatile hypergolic fuels can be extremely dangerous. In his book Empire of the Clouds, author James Hamilton Paterson relates the grisly aftermath of a World War II rocket-powered airplane crash. "The Komet's fulminating rocket fuels would explode spontaneously when mixed in even minute quantities and on one hideous occasion had actually dissolved a pilot alive before rescuers could free him from his crashed aircraft," he writes.

Since those early days, aerospace engineers have learned a lot about hypergolic rocket motor safety and SpaceX will no doubt have the tanks well protected.